It is well known in the art of photographic science to add dyes to the structure of a photographic element in such a way as to absorb deleterious light. The dye can be located in several positions including: between the exposure source and the photosensitive emulsion; between the photosensitive emulsion and the support; and on the opposite side of the support to inhibit halation. Specific applications are illustrated, for example, in Research Disclosure, No. 308, December 1989, Item 308119.
Antihalation is particularly important when a film is exposed with a laser such as in the use of long wavelength laser films for medical imaging. The high intensity of long wavelength light (i.e. red to infrared) emitted by the laser is difficult to filter completely. If high levels of a dye are used it is typically difficult to remove the dye during processing and the result is a coloration, or dye stain, which is undesirable.
Dyes which are useful as antihalation dyes are taught in U.S. Pat. No. 4,839,265. The dyes correspond to the formula: ##STR2## wherein; L represents a substituted or unsubstituted methine groups wherein the substitution includes substituted or unsubstituted lower alkyl groups containing from 1 to 5 carbon atoms, halogen atoms, substituted or unsubstituted aryl groups and lower alkoxy groups.
These dyes are manufactured either from relatively expensive ingredients, or from ingredients which are difficult to prepare. Typically, these dyes are manufactured as follows: ##STR3## This process uses chlorodinitrobenzene and generates dinitroaniline, which both have deleterious effects on photographic elements. Furthermore, only symmetrically-substituted dyes can be prepared by this process.
U.S. Pat. No. 4,876,181 teaches the use of antihalation dyes which correspond to the following structure: ##STR4## wherein R.sub.3 and R.sub.5 are each hydrogen or together represent the atoms necessary to complete a 5- or 6-membered carbocylic ring. These compounds are also prepared from expensive ingredients or from ingredients which are difficult to prepare. For example, dyes of U.S. Pat. No. 4,876,181 can be prepared as described in U.S. Pat. No. 4,839,265 or by a more elaborate process illustrated generally below: ##STR5## Dyes prepared by this process are considerably more expensive than dyes prepared by simpler processes. There has been a long felt need in the art for a dye which can be obtained from a simpler preparation procedure.
There has been a desire in the art to provide a dye which is easily prepared and which absorbs at long wavelengths (i.e. red to infrared) and which does not impart a color to the processed film.